You can follow these steps:
1. Sign in to Blogger.
2. Select the blog to update.
3. In the left menu, click Settings > Basic > Publishing.
4. Under your current domain, click Set up a 3rd party URL for your blog.
5. Enter your URL, including the www.
6. Click Save.
7. An error will appear with two CNAMEs.
8. Visit your domain registrar's website and find the DNS (Domain Name System) or the Name Server settings in the control panel.
9. Delete existing CNAME entries.
10. Enter the CNAMEs provided in the error message and click Save.
11. Wait for a few hours for your settings to activate, then follow steps 1–6 above.